Japanese Patent Application Publication No. 11-179673 discloses a tightening tool for shutting off torque transmission to a fastener when the tightening torque of the fastener reaches a predetermined value. In this tightening tool, the rotary torque of a motor is transmitted to a main shaft via a clutch, and the fastener is tightened by rotating the main shaft. The clutch used in this type of tightening tool comprises a pair of mutually opposed clutch members and biasing means (e.g., a spring) for pressing one of the clutch members toward the other, for example. When the tightening torque of the fastener (i.e. the load acting on the main shaft) is lower than the predetermined value, the clutch members are maintained in an engaged state such that the torque of the motor is transmitted to the main shaft. On the other hand, when the tightening torque of the fastener reaches or exceeds the predetermined value, the state of engagement between the clutch members is released, and torque transmission from the motor to the main shaft is shut off.